The present invention relates to an image reading system that reads a color image formed on an image sensor through an imaging optical system including a diffractive grating.
An image input device such as an image scanner or a digital facsimile includes an image reading system that comprises an imaging optical system and an image processing device. An image of the object is formed on an image sensor such as a CCD through the imaging optical system, the image processing device process the image data from the CCD to generate a picture signal such as an RGB component signal.
Recently, the resolution of the input device is developed to take a clearer image without increasing the device size. Increased resolution of the input device requires an increase of density of pixels in the CCD. This also requires a high resolution (low aberration) lens in the imaging optical system.
The imaging optical system may employ a diffractive grating in addition to the refractive lenses to compensate chromatic aberration. Since the dispersion of the diffractive grating has opposite sign to that of the refractive lenses, a use of the diffractive grating reduces the chromatic aberration without increasing a number of elements.
The diffractive grating diffracts and separates incident light into various order diffractive lights. When the diffractive grating is used instead of the refractive lens, the diffractive grating is designed so as to maximize a diffraction efficiency of the predetermined order diffractive light. The diffraction efficiency is a ratio of intensity of the predetermined order diffractive light to that of the incident light. In general, the grating is designed to maximize the intensity of the first-order diffractive light.
However, the diffraction efficiency varies according to the wavelength of the used light. An increase of a difference between the wavelength of the used light and the designed wavelength decreases the diffraction efficiency. For example, when the diffractive grating is optimized (blazed) at wavelength of 525 nm, the diffraction efficiency at 525 nm is 100% while that at 436 nm is 87%, and that in 656 nm is 88%. That is, the intensity of the first-order light decreases and the intensity of the other order light increases.
Since any order light except the first-order has different convergence from that of the first-order light, the decrease of the diffraction efficiency causes flare that deteriorates quality of the image.
Japanese laid-open patent publication No. Hei 9-238357 discloses an image reading system that employs an image processing unit to eliminate flare component due to the use of a diffractive grating.
The imaging optical system of the publication comprises nine lenses and a diffractive element. The diffractive element, which is a plane parallel plate on which a diffractive grating is formed, is arranged between the ninth lens and the CCD. On the other hand, an aperture stop is disposed between the fourth lens and the fifth lens.
In the above construction, since the extent of the blurred spot of the unnecessary order diffractive light (flare component) varies depending on the position of the target pixel on the CCD, the image processing unit should calculate the effect of the flare component with considering the position of the target pixel.
However, such the process increases a load of the processing unit because of enormous calculations.
It is therefore an object of the present invention to provide an image reading system to reproduce a clearer image with decreasing a load of the processing unit as compared with the conventional system.
According to a first aspect of the invention, an image reading system includes an imaging optical system that includes at least one refractive lens and a diffractive grating blazed at a predetermined wavelength, the optical system forming an image of an object by a predetermined order diffractive light, an aperture stop positioned close to said diffractive grating, a main image sensor for receiving the images of respective color components, and a flare compensating unit for compensating original image signals from the main image sensor to eliminate flare components due to unnecessary order diffractive light except the predetermined order diffractive light.
With this construction, the flare components are eliminated by only calculating the original image signals. Further since the aperture stop is close to the diffractive grating, the blurred spot of the unnecessary order diffractive light will be constant regardless of the position on the image plane. This decreases a load for the calculating unit because the position of a target pixel under compensation is not required to be taken in the flare compensation.
The diffractive grating may be formed on the surface of the refractive lens.
The compensating means may compensate the original image signals of the color components except the color component including the blazed wavelength.
In general, the color components are R (red), G (green) and B (blue) and the blazed wavelength is included in the G component. In such the case, the compensating means compensates the original image signals of the R and B component.
Further, the compensating means may compensate the original image signals of a target pixel based on the original image signals of a surrounding pixels within a predetermined image area. According to the first aspect, the extent of said image area can be considered as constant regardless of the position of the target pixel.
According to the second aspect, the image reading system includes an imaging optical system that includes at least one refractive lens and a diffractive grating blazed at a predetermined wavelength, the imaging optical system forming an image of an object by a predetermined order diffractive light, a main image sensor for receiving the image of respective color components, at least one auxiliary image sensor located at a defocus position being different from an equivalent plane with the main image sensor to output average intensity signals of the predetermined color component, a beam splitter for dividing light from the object through the imaging optical system between the main image sensor and the auxiliary image sensor, and means for compensating the image signals of the target pixel of the main image sensor using the average intensity signals of the pixel corresponding to the target pixel in order to eliminate flare components due to unnecessary order diffractive light except the predetermined order diffractive light.
With this construction, the auxiliary image sensor detects the optically averaged light due to the defocus arrangement, the compensation means is not required for the averaging calculation. It reduces the load of the compensation means.
According to the third aspect of the present invention, the image reading system includes an imaging optical system that includes at least one refractive lens and a diffractive grating blazed at a predetermined wavelength, the optical system forming an image of an object by a predetermined order diffractive light, a main image sensor for receiving the images of respective color components, at least one light receiving element for receiving the light from whole reading area of the object to output total intensity signal, and means for compensating the image signals of the target pixel of the main image sensor using the total intensity signal from the light receiving element in order to eliminate flare components due to unnecessary order diffractive light except the predetermined order diffractive light.
With this construction, the compensating means repeats the identical correction for all of the effective pixels based on the total intensity signal. This process is effective to a document.